1. Field of the Invention
The present invention relates to a projection exposure apparatus used to transfer a mask pattern onto a photosensitive substrate by a lithography process for manufacturing devices such as semiconductor elements, image pickup elements (such as CCDs), liquid crystal display elements or thin film magnetic heads, a cleaning method and a maintenance method for a liquid immersion type projection exposure apparatus, and a device manufacturing method.
2. Description of Related Art
A projection exposure apparatus is used that, when a semiconductor element, etc., is manufactured, transfers the pattern image of a reticle as the mask via a projection optical system to the respective shot regions on a wafer (or glass plate, etc.) that has been coated with resist as the photosensitive substrate. Conventionally, step and repeat system reduction projection type exposure apparatuses (steppers) have been widely used as projection exposure apparatuses, but recently step and scan system projection exposure apparatuses that perform exposure by simultaneously scanning the reticle and the wafer have also been receiving attention.
The shorter the exposure wavelength used is, and the larger the numerical aperture of the projection optical system used is, the higher the resolution of the projection optical system provided on the projection exposure apparatus becomes. For this reason, the exposure wavelength used in the projection exposure apparatus has become shorter year by year in association with miniaturization of integrated circuits, and the numerical aperture of the projection optical system is also increasing. In addition, the mainstream exposure wavelength at present is the 248 nm of a KrF excimer laser, but a shorter wavelength, the 193 nm of an ArF excimer laser, is also coming into practical application.
In addition, when exposure is performed, the depth of focus (DOF) is also important as well as the resolution. The resolution R and the depth of focus δ are expressed by the respective equations below.R=k1·λ/NA,  (1)δ=±k2·λ/NA2,  (2)
Here, λ is the exposure wavelength, NA is the numerical aperture of the projection optical system, and k1 and k2 are process coefficients. Based on Equation (1) and Equation (2), it is apparent that when the exposure wavelength λ is made shorter and the numerical aperture NA is made larger in order to enhance the resolution R, the depth of focus δ becomes narrower. Conventionally, in projection exposure apparatuses, exposure has been performed by aligning the surface of the wafer with the image plane of the projection optical system using an autofocus system, but for this purpose it is necessary that the depth of focus δ be broad to a certain extent. Therefore, in the past as well, there have been proposals to essentially broaden the depth of focus, such as the phase shift reticle method, the modified illumination method, and the multilayer resist method.
As mentioned above, in conventional projection exposure apparatuses, the depth of focus is becoming narrower due to shorter wavelength of the exposure light and larger numerical aperture of the projection optical system. In addition, in order to handle further high integration of semiconductor integrated circuits, even shorter exposure wavelengths are being researched, and with the current situation there is concern that the depth of focus will become too narrow, and the margin during the exposure operation will be insufficient.
Therefore, the liquid immersion method has been proposed as a method of essentially shortening the exposure wavelength and broadening the depth of focus. In liquid immersion type projection exposure apparatuses that use this liquid immersion method, the space between the lower surface of the projection optical system and the wafer surface is filled with a liquid such as water or an organic solvent, and the fact that the wavelength of the exposure light in liquid becomes 1/n in air (n is normally approximately 1.2 to 1.6 at the refractive index of the liquid) is used to increase the resolution while expanding the depth of focus (for example, see Japanese Unexamined Patent Application, First Publication No. H10-303114, and PCT International Publication No. 99/49504).
In liquid immersion type projection exposure apparatuses, there is a possibility that impurities dissolved in the liquid will precipitate and accumulate in the piping and/or at the front end portion of the projection optical system, which comes into contact with the liquid, leading to deterioration of performance as a liquid immersion type exposure apparatus. Particularly, the fluorite (CaF2) and barium fluoride (BaF2), for example, that can be used as the lens material at ultraviolet wavelengths may be eroded, if purified water or an aqueous solution is used as the liquid due to the solubility thereof, and the concentration of calcium ions and barium ions, known as the scale component, in the liquid increases, and, as a result, due to precipitation and accumulation of the scale, there is a possibility that deterioration of performance as a liquid immersion type exposure apparatus would accelerate.
In addition, minute solid matter consisting of metal salts such as calcium, magnesium, iron, nickel, chrome, etc., and minute solid matter consisting of resins such as PTFE (polytetrafluoroethylene) may be included in the liquid used as the liquid immersion type projection exposure apparatus. These are also extremely low quantity impurity components included in the liquid itself, and are also components that form the piping, etc., that form the flow paths through which the liquid flows.
These components consisting of metal salts accumulate and grow on the walls, etc., of the liquid flow path over a long period of time and become an extremely small solid matter, and when growth proceeds further, it peeled and fell off from the walls of the liquid flow path, and it mixed into the liquid as minute solid matter. PTFE, which is considered to be relatively stable, can be collided by the minute solid matter consisting of metal salts and can itself become new minute solid matter.